Left axis deviation in patients with non-ischemic heart failure and left bundle branch block is a purely electrical phenomenon.

Abstract Background Possible mechanisms of left axis deviation (LAD) in the setting of left bundle branch block (LBBB) include differences in cardiac electrophysiology, structure, or anatomical axis. Objectives We sought to clarify the mechanism(s) responsible for LAD in patients with LBBB. Methods Twenty-nine patients with non-ischemic cardiomyopathies and LBBB underwent non-invasive electrocardiographic mapping (ECGi), cardiac computed tomography, and magnetic resonance imaging in order to define ventricular electrical activation, characterize cardiac structure, and determine the heart anatomical axis. Results Sixteen patients had a normal QRS axis (NA, mean axis: 8±23°) whereas 13 patients had LAD (mean axis: -48±13°, p 2 =0,67, p Conclusion Left axis deviation in left bundle branch block appears to be due to electrophysiological abnormalities rather than structural factors or the cardiac anatomical axis.